Insulated pipe structure and methods of making such structures

ABSTRACT

An insulated pipe structure suitable for use in an undersea pipeline comprises, intermediate its ends, an inner pipe ( 11 ) for serving as a conduit for fluid flowing through the pipe structure, thermally insulating material ( 3 ) extending around the inner pipe and an outer shell ( 2 ) extending around the insulating material ( 3 ). At each of the ends of the pipe structure the inner pipe is connected to the outer shell by a thermally insulating connecting element ( 4 ) which is secured to the inner pipe ( 11 ) at least partly by virtue of adhesive forces between the inner pipe ( 11 ) and the connecting element ( 4 ). The thermally insulating connecting element ( 4 ) and the outer shell ( 2 ) together protect the insulating material ( 3 ) from water ingress and from ambient pressure.

This invention relates to an insulated pipe structure suitable for usein an undersea pipeline, to an insulated pipeline comprising amultiplicity of such pipe structures and to methods of forming such apipe structure and such a pipeline.

As is well known, there is a requirement in the offshore oil and gasindustry for a pipeline that can be laid on the seabed and is insulatedso that the raw fluids extracted from a well can be transported throughthe pipeline without excessive cooling of those fluids. As a resultthere have been many proposals for pipe structures suitable for use insuch pipelines.

Most commonly a pipe structure for use in such a pipeline comprises,intermediate its ends, an inner pipe for serving as a conduit for thefluid transported by the pipe structure, insulating material around thepipeline and an outer pipe around the insulating material. The pipestructure is formed in a discrete length, for example of the order of 12m and a pipeline is formed by joining end-to-end a multiplicity of pipestructures. It is usually important that in the completed pipeline thenature of the joints between adjacent pipe structures is such that theinsulating material is protected from ingress of seawater and from theambient pressure at the seabed, in order to preserve its thermalinsulating properties.

Finding a satisfactory pipe structure has proved difficult, especiallyif the pipeline may be laid in deep water. A particular challenge arisesbecause of the desirability of being able to join one length of pipestructure to another during laying of the pipeline. This may be done ona vessel laying the pipeline and the rate at which the laying is able tobe carried out may be determined by the time taken to join one pipestructure to another. At the same time, during laying of the pipeline,especially if the J-laying technique is adopted, there is likely to be aneed to grip the pipe structure and apply substantial longitudinalforces to the exterior surface of the pipe structure. In order totransfer the longitudinal forces that arise during J-laying it is usualpractice to weld adjacent ends of the outer pipes together, eitherdirectly or indirectly, as well as welding the inner pipes together. Inthat case, the longitudinal upward force applied to the uppermost outerpipe during laying can be transferred down the pipeline along the outerpipe and only transferred to the inner pipe over a multiplicity oflengths of the pipe structure. In that case the longitudinal shear forcethat has to be transferred between the outer pipe and the inner pipeover one length of the pipe structure is limited. A problem that arises,however, is that because of the need to weld together both the innerpipes and the outer pipes, the welding operation is a time consumingprocess.

WO 98/17940 describes a pipeline construction of the kind just referredto. In that case, outer pipes are securely joined together bycylindrical covers that are welded over the adjacent ends of the outerpipes. There is still a need in such a construction to provide somemechanism for the transfer of shear forces between the inner and outerpipes but because the outer pipes are joined by welding the shear forcesthat have to be transferred between the inner and outer pipes is muchless. WO 98/17940 refers to the possibility of transferring up to threetonnes between the inner and outer pipes. That force is transferred byelastomeric bulkheads extending between the inner and outer pipes, eachbulkhead being formed by an elastomeric member provided in the spacebetween the pipes and longitudinally compressed by a suitable mechanicalarrangement so as to be pressed radially against the inner and outerpipes. As already indicated, in such an arrangement welding operationshave to be carried out on both the inner pipes and the outer pipes whena joint is formed. A further problem with arrangements of the kinddescribed is that they are complicated and time consuming to installand, in order to have a chance of being effective, the additional radialstresses introduced into the inner and outer pipes as a result of theradial pressure of each bulkhead must be substantial.

Once a pipeline has been laid temperature differences between the innerpipe exposed to the relatively hot well fluids and the exterior of thepipeline exposed to the relatively cold sea are likely to lead tolongitudinal shear stresses as a result of the different thermalexpansions of interior and exterior parts of the pipeline. Thus evenafter laying, significant longitudinal shear forces between the innerand outer pipes have to be accommodated, although the forces aresubstantially less than the ones that occur during J-laying.

GB 2161565A describes a pipe structure that comprises, intermediate itsends, inner and outer concentric pipes with thermally insulatingmaterial therebetween. At each end of the pipe structure an annularconnecting member is provided, the wall of the connecting member havinga longitudinal section that is generally Y shaped with each of the upperpair of limbs of the Y being butt welded to respective ones of the innerand outer pipes and the single lower limb of the Y providing an end ringto the pipe structure. The end ring of one pipe structure can readily bebutt welded to the end ring of another pipe structure thereby enablingpipe structures to be joined securely together end-to-end. Theconnecting member and the inner and outer pipes are typically made ofsteel.

The structure of GB 2161565A succeeds in providing a pipe structure thatis easy to join end-to-end with another similar structure, in which themechanical connection between the inner and outer pipes is exceptionallystrong and in which the insulating material provided between the innerand outer pipes is protected from ambient pressure and from ingress ofwater. However, one disadvantage of such a design is that the connectingmember provides a thermal bridge between the inner pipe and the outerpipe seriously affecting the extent to which the fluids travellingthrough the inner pipe are thermally insulated from the colder sea.

GB 2191842A describes a similar proposal with a similar disadvantage.

It is an object of the invention to provide a pipe structure whichovercomes or mitigates the disadvantages referred is above.

According to the invention there is provided a pipe structure suitablefor use in an undersea pipeline, the pipe structure being elongate andcomprising, intermediate its ends, an inner pipe for serving as aconduit for fluid flowing through the pipe structure, thermallyinsulating material extending around the inner pipe and an outer shellextending around the insulating material, wherein at each of the ends ofthe pipe structure the inner pipe is connected to the outer shell by athermally insulating connecting element which is secured to the innerpipe at least partly by virtue of adhesive forces between the inner pipeand the connecting element, the thermally insulating connecting elementand the outer shell together protecting the insulating material fromwater ingress and from ambient pressure.

By using a thermally insulating connecting element which relies at leastpartly on adhesive forces to secure the inner pipe and the connectingelement, it becomes possible in a simple way to have a pipe structurewhich can easily be joined end-to-end to another similar structure onlyby welding the inner pipe, which can protect the insulation around theinner pipe both from ambient pressure and from ingress of water, andwhich can accommodate substantial longitudinal loads (commonly 150tonnes or more) applied between the inner pipe and the outer shell, forexample during J-laying. At the same time, because the connectingelement is thermally insulating, it does not act as a thermal bridge inthe manner of GB 2161565A or GB 2191842A.

The outer shell of the insulated pipe structure is of sufficientstrength and rigidity to protect the insulating material around theinner pipe from the high ambient pressure on the seabed. Thus the outershell is preferably substantially rigid. The outer shell may be apre-formed tubular member, for example a steel pipe. In that case theconnecting element is preferably secured to the pre-formed tubularmember at least partly by virtue of adhesive forces between the tubularmember and the connecting element. The connecting element preferablyengages an inner surface of the tubular member and preferably engages anouter surface of the pipe. Thus the connecting element may be in theform of an annular member filling the annular space between end portionsof the inner pipe and the tubular member.

Reference is made above to the securing of the connecting element “atleast partly by adhesive forces.” In embodiments of the inventiondescribed below the connecting element is secured predominantly byadhesive forces and that is generally preferred but it should beunderstood that a portion of the strength of the connection may bederived from other effects. For example, the effect of water pressuremay be to generate radial forces between the connecting element and theinner pipe and between the tubular member and the connecting element,and those forces may give rise to significant friction forces which cansupport the adhesive forces in resisting any longitudinal movement ofthe inner pipe relative to the outer shell. In order to promote goodadhesion the surface of the inner pipe and/or the surface of the tubularmember to which the connecting element is secured may be surface-treatedand, indeed, some radially extending projections/recesses may beprovided on those surfaces to improve further the securing of theconnecting element. Also it is possible to arrange for example, for theconnecting element to expand in situ as a result of a chemical change inorder to introduce radial forces and therefore friction forcesstrengthening the connection.

Preferably the connecting element is formed in situ. The element may forexample be composed entirely of material that is generally regarded asan adhesive.

Instead of providing a pre-formed tubular member for the outer shell anda separate connecting element, the connecting element may be formed asan integral part of the outer shell and the connecting element and theouter shell may be formed in situ. An example of an arrangement of thiskind and the manner in which it can be made is described below withreference to the drawings.

Preferably, the inner pipe projects in an axial direction from oppositeends of the pipe structure. That facilitates connection of one pipestructure to another because access to the junction of the inner pipesis easily obtained.

The connecting element may be formed from various materials that areknown per se. Especially in the case where the outer shell is apre-formed tubular member and the connecting element fills an annularspace between end portions of the inner pipe and the tubular member, theconnecting element may be an unexpanded material, for example asynthetic polymeric material such as polyurethane or some other resinmaterial. In an example of the invention described below the materialcomprises a polyol isocyanate. Especially in the case where the outershell is integrally formed with the connecting element and the casewhere the connecting element extends over the outer shell, theconnecting element may be formed from a composite material that may forexample comprise a filler material such as glass fibre, carbon fibre orKevlar (registered trade mark) in a polymeric resin material which maybe an unexpanded material and may for example be polyurethane.

The axial length of connection between the connecting element and theinner pipe and, if there is one, the connection between the connectingelement and the outer shell can be chosen so as to enable sufficientshear forces to be transferred through the connection.

It will be noted that both the material placed between the inner pipeand the outer shell intermediate its ends and the material of theconnecting element are described as thermally insulating materials. Itshould be understood, however, that the material of the connectingelement does not need to have such good insulating properties as thematerial placed between the inner pipe and the outer shell intermediateits ends, because the cross-sectional area of the thermal path throughthe connecting element is so much smaller over a full length of the pipestructure. Thus, the thermal conductivity of the material forming thethermally insulating connecting element is preferably less than 1.00 W/°Km and more preferably less than 0.2 W/° Km. The insulating materialplaced between the inner pipe and the outer shell preferably has athermal conductivity less than not only 1.00 W/° Km, but also less than0.2W/° Km and indeed preferably substantially less than those values;for example, the thermal conductivity of the insulating material ispreferably less than 0.1 W/° Km. In order that the insulating materialbetween the outer shell and the inner pipe is as effective as possibleit preferably occupies over 90 percent, preferably substantially all, ofthe space between the shell and the pipe and bounded at its ends by theconnecting elements. In the event that not all the space is occupied bythermally insulating material or not all by the same thermallyinsulating material, the thermal insulation provided between the innerpipe and the outer pipe is preferably more effective than that of amaterial filling the space and having a thermal conductivity of 0.1 W/°Km.

As has already been described, during laying of a pipeline high demandsare placed on the pipe structure to withstand longitudinal shear forcesbetween the inner pipe and the outer shell. Preferably the strength ofthe connection between the outer shell and the inner pipe is such thatit can withstand a longitudinal shear force of more than 75 tonnes. Inthat case a total longitudinal force of more than 150 tonnes can betransferred from the outer pipe to the inner pipe by a single pipestructure, as may be required when J-laying, especially in deep water.There may of course be some contribution to the overall longitudinalshear force between the inner pipe and the outer shell that thestructure can withstand from other parts of the structure such as theinsulating material placed between the inner pipe and the outer shellintermediate the ends of the pipe structure, but that is unlikely tocontribute significantly in the case of most insulating materials.

The pipe structure may comprise a single pipe length comprising a singleinner pipe and a single outer shell but generally it will be preferredthat the pipe structure comprises a plurality of pipe lengths, eachlength comprising an inner pipe, insulating material and an outer shell,the pipe lengths being connected end-to-end. Whilst it is possible forthermally insulating connecting elements to be provided at the joints ofthe pipe lengths intermediate the ends of the pipe structure, it willusually be preferred for those joints to be of another design which maybe well known per se; those intermediate joints can be made on shore andthe procedure required to form them need not have any effect on thesubsequent pipe laying procedure.

The present invention also provides an insulated pipeline comprising amultiplicity of insulated pipe structures as defined above, the pipestructures being joined end-to-end.

The pipe structures are most advantageously structurally connected toone another at least principally by virtue of the connections of theends of the inner pipes of the pipe structures. It will be appreciatedthat the insulated pipeline formed from the pipe structures isdistinctive because not only is there not a continuous metal outer pipeconsisting of lengths of outer pipe connected together by welding (asthere is in for example WO 98/17940) but nor is there a continuous outermetal structure (as there is in for example GB 2161565A where lengths ofsteel outer pipe are joined together by joints which connect them to theinner pipe). Rather in the present insulated pipeline, the outer pipemay be formed of lengths of metal pipe interrupted at joints of adjacentpipe structures by connecting elements of thermally insulating materialor the outer pipe may be formed entirely of thermally insulatingmaterial. The ends of the inner pipes of adjacent pipe structures arepreferably welded together. At the connection of one pipe structure toanother, a thermally insulating arrangement is preferably providedaround the adjacent inner pipe ends between the adjacent connectingelements. As will be understood, that arrangement need not have any loadbearing capability. Whilst it is of course desirable for the insulatingmaterial of that thermally insulating arrangement to be protected fromwater ingress and from ambient water pressure by the arrangement, itwill be appreciated that because of the limited longitudinal extent ofthe arrangement such matters are not so critical.

The present invention also provides a method of forming an insulatedpipe structure as defined above. The outer shell may comprise apre-formed tubular member and the connecting element may be formed insitu; alternatively the outer shell and the connecting element may beformed in situ during the same operation and may be integral with oneanother.

The present invention further provides a method of forming an insulatedpipeline by joining together end-to-end a multiplicity of pipestructures, the pipe structures being as defined above. The method ispreferably carried out during laying of the pipeline; thus, the methodpreferably includes the steps of laying the pipeline undersea from apipe-laying vessel and joining the pipe structures successively to anend of the pipeline as the pipeline is laid. The method is especiallysuitable in the case where the pipeline is laid using a J-lay techniquebecause of the magnitude of the longitudinal shear forces that may beapplied during J-laying.

By way of example certain embodiments of the invention will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 is a sectional side view of an end of a pipe structure embodyingthe invention;

FIG. 2 is a side view of a pipe structure made up of four sectionsjoined together end-to end, the ends of the pipe structure being of theform shown in FIG. 1;

FIG. 3 is a sectional side view of an end of another pipe structureembodying the invention;

FIG. 4 is a side view of a pipe structure made up of four sectionsjoined together end-to-end, the ends of the pipe structure being of theform shown in FIG. 3; and

FIG. 5 is a sectional view of a modified form of pipe structure.

Referring first to FIG. 1, the pipe structure shown generally comprisesan inner pipe 1, an outer pipe 2 and thermally insulating material 3.The right hand end (right hand as seen in FIG. 1) is provided with aconnecting element 4 which is of annular shape and extends it betweenthe inside of the outer pipe 2 and the outside of the inner pipe 1. Ascan be seen in FIG. 1, the outer pipe 2 ends in substantially the sameposition along the longitudinal axis of the structure as the element 4,but the inner pipe 1 passes through the element 4 and projects a shortdistance beyond the element 4, the projecting part being indicated byreference numeral 11.

The inner and outer pipes 1 and 2 are of materials and dimensions thatare known per se. For example the inner and outer pipes may be formed ofsteel while the thermally insulating material 3 may be any of thethermally insulating materials conventionally used in such structures. Arange of such materials is described in WO 98/59193. The externaldiameter of the inner pipe 1 in one particular example is about 220 mm(8 inch pipe) and the external diameter of the outer pipe 2 in thatexample is about 324 mm (12 inch pipe).

The connecting element 4 is made of a thermally insulating materialwhich provides much more of a thermal barrier than, say, a thermallyconducting material such as steel, but need not be as good a thermalinsulator as the thermally insulating material 3 because of the morelimited longitudinal extent of the element 4. The connecting element 4is also required to have substantial adhesive properties so as toprovide a sufficiently strong bond with both the inner pipe 1 and theouter pipe 2 to resist the longitudinal shear forces that may be appliedto those bonds during use. In order to promote good bonding of theelement 4 to the pipes 1 and 2 the areas of the pipe surface to whichthe element 4 is required to bond may be pre-treated, for example bysand-blasting, to create a roughened surface finish.

In the example being described, the connecting element 4 is formed insitu and is a polyol isocyanate having a thermal conductivity of about0.16 W/° Km. If desired, an annular shutter (not shown) may be placedbetween the inner and outer pipes immediately to the outside of theinsulation 3 and employed to limit the axially inward penetration of thematerial being formed into the element 4.

Although it is possible for the pipe structure, whose end is shown inFIG. 1, to be made up of a single length of inner pipe and a singlelength of outer pipe joined at their other ends in the same way as isillustrated in FIG. 1, that is not usually the preferred construction.FIG. 2 illustrates schematically the preferred approach in which thepipe structure is made up of a plurality of lengths of inner and outerpipes (in this particular example four lengths of each pipe). The pipestructure shown in FIG. 2 would usually be manufactured as the unitshown on dry land and thereafter transferred to a pipe-laying vessel orthe like whose successive pipe structures would be joined end-to-end toform a pipeline laid from the vessel.

In FIG. 2 the four lengths of inner pipe are referenced 1A, 1B, 1C and1D, whilst the four lengths of outer pipe are referenced 2A, 2B, 2C and2D. It will be understood that each of the ends of the pipe structureshown in FIG. 2 are of the form shown in FIG. 1 so that the structure isprovided with inner pipe projecting parts 11A and 11D at its oppositeends. The length of the projecting ends are exaggerated in the interestsof clarity in FIG. 2.

The intermediate joints between the adjacent ends of four lengths of theinner and outer pipes may be formed in the same way as described inrespect of FIG. 1, but that may not be necessary, especially if theassembly of individual pipe lengths into the structure shown in FIG. 2can take place prior to pipe-laying and therefore outside the criticalpath. A simple and satisfactory intermediate joint may be made byarranging for the outer pipe lengths each to be shorter than the lengthsof inner pipe. Adjacent lengths of inner pipe can then be butt weldedtogether with an axial gap formed around the inner pipe ends. Thatannular gap can be closed in a watertight and pressure resistant mannerby welding two semi-circular rings 7 to the outer pipe and to each otherafter placing any further insulating material that may be desirablearound the inner pipe ends. Although the operation of joining the innerand outer pipe lengths using the rings 7 involves a series of weldingsteps and is therefore time consuming, the extra time required does notrepresent a particular disadvantage if the welding steps are carried outin advance of the laying operation and do not therefore delay the layingof the pipe.

Of course the number of pipe lengths joined end-to-end to form the pipestructure shown in FIG. 2 may be varied as may the length of each pipelength. In a particular example of the invention the pipe structure isformed from four lengths of inner and outer pipes and has an overalllength of 48 m.

Typically, the pipe structure is laid by a vessel employing a S-lay orJ-lay technique. First the pipe structure as shown in FIG. 2 is broughtinto end-to-end coaxial relationship with the end of the pipeline beinglaid, that end being of the form shown in FIGS. 1 and 2 (as a result ofprevious pipe structures having been laid). The end 11 of the inner pipe1 of the pipe structure to be laid is then butt welded to the end of theinner pipe of the pipeline. Insulation can then be placed around thebutt welded ends of the inner pipe and covered with a shield in anyappropriate manner. It should be noted that, in the case of theinsulation around the inner pipe joint just referred to, it is not soimportant to protect the insulation from the ingress of water and/orambient pressure because of the limited longitudinal extent of thejunction. The joining of the pipe structure to the end of the pipelinecan therefore be accomplished in a relatively simple and rapid manner.

Especially in the case where the pipeline is laid by J-laying asubstantial upward longitudinal force will be applied to the exterior ofthe pipe structure just after it has been attached to the pipeline. Thatforce is transmitted via the connecting elements 4 of the pipe structureto the inner pipe and thus to the pipeline as a whole extendingdownwardly from the vessel.

After laying there may still be substantial longitudinal shear forcesbetween the inner and outer pipes 1, 2 arising from their temperaturedifferences and the thermal expansion properties of the two pipes. Thoseshear forces are likely to be lower than the forces experienced duringJ-laying but may still be substantial. Since the greatest shear forcesare experienced during laying it may not be serious if the adhesiveforces between the connecting elements 4 and the inner and/or outerpipes 1, 2 deteriorate to some extent over time.

FIG. 3 shows a pipe structure of generally similar form to the structureas shown in FIG. 1. In this description corresponding parts aredesignated by the same reference numerals and only features of thestructure of FIG. 3 that differ from the structure of FIG. 1 aredescribed below. In the arrangement shown in FIG. 3, the connectingelement 4 is bonded to the outside of the outer pipe 2 instead of theinside; the connecting element is again formed in situ and can be formedby mounting the assembly of the inner pipe 1, outer pipe 2 andinsulating material 3 on a mandrel or some other appropriate rotatablemounting, and winding a filament loaded with suitable adhesive aroundthe end of the assembly to form an end to the pipe structure of the formshown in FIG. 3.

FIG. 4 is a view similar to FIG. 2 and showing a pipe structure of theform described above with reference to FIG. 2, but incorporating the endarrangement shown in FIG. 3 rather than FIG. 1. Corresponding parts aredesignated by the same reference numerals in FIGS. 2 and 4.

It will be understood that the pipe structure illustrated in FIGS. 3 and4 is used in substantially the same way as already described for thepipe structure of FIGS. 1 and 2.

FIG. 5 shows a pipe structure that is a modified form of the structureshown in FIGS. 3 and 4. Instead of forming only the connecting elementby winding a filament loaded with suitable adhesive, the whole of theouter shell, indicated by reference numeral 22 in FIG. 5, and theconnecting elements at each end of the structure, indicated by referencenumeral 24 in FIG. 5, are formed integrally with one another during sucha filament winding process. Thus, rather than having inner and outersteel pipes, the structure comprises an inner pipe, which may be steel,and an outer shell which is formed of a synthetic polymeric material.The outer shell must of course be sufficiently thick to withstand theambient pressures to which the structure is to be subjected on theseabed. It will be appreciated that in the case of the structure of FIG.5 it may be appropriate to have all junctions between pipe lengths ofthe same kind and it may be advantageous to form the structure of FIG. 5at a substantial unit length (for example 48 m), and to form all thejoints during laying of the pipeline.

As has been indicated above it is important for joints between theconnecting element and the inner pipe and also, if there are any, jointsbetween the connecting element and the outer pipe to be as watertight aspossible. If desired, the watertightness of such joints, especially thewatertightness over an extended period of time, may be enhanced byincorporating one or more ‘O’ ring seal arrangements into the joints.

1. An insulated pipe structure suitable for use in an undersea pipeline,the pipe structure being elongate and comprising, intermediate its ends,an inner pipe for serving as a conduit for fluid flowing through thepipe structure, thermally insulating material extending around the innerpipe and a substantially rigid outer shell extending around theinsulating material, wherein at each of the ends of the pipe structurethe inner pipe is connected to the outer shell by a thermally insulatingconnecting element which is secured to the inner pipe at least partly byvirtue of adhesive forces between the inner pipe and the connectingelement, the thermally insulating connecting element is a unitary onepiece element, and the thermally insulating connecting element and theouter shell together protecting the insulating material from wateringress and from ambient pressure.
 2. An insulated pipe structureaccording to claim 1, in which the outer shell comprises a pre-formedtubular member.
 3. An insulated pipe structure according to claim 2, inwhich the connecting element is secured to the pre-formed tubular memberat least partly by virtue of adhesive forces between the tubular memberand the connecting element.
 4. An insulated pipe structure according toclaim 2, in which the connecting element engages an inner surface of thetubular member.
 5. An insulated pipe structure according to claim 2, inwhich the connecting element engages an outer surface of the tubularmember.
 6. An insulated pipe structure according to claim 1, in whichthe connecting element is formed in situ.
 7. An insulated pipe structureaccording to claim 1, in which the inner pipe projects in an axialdirection from opposite ends of the pipe structure.
 8. An insulated pipestructure according to claim 1, in which the connecting elementcomprises a synthetic polymeric material.
 9. An insulated pipe structureaccording to claim 8, in which the synthetic polymeric material is athermosetting resin.
 10. An insulated pipe structure according to claim1, in which the thermal conductivity of the material forming thethermally insulating connecting element is less than 1.0 W/Km.
 11. Aninsulated pipe structure according to claim 1, in which the thermallyinsulating material extending around the inner pipe occupiessubstantially all the space between the outer shell and the inner pipeand bounded at its ends by the connecting elements.
 12. An insulatedpipe structure according to claim 1, in which the structure comprises aplurality of pipe lengths, each length comprising an inner pipe,insulating material and an outer shell, the pipe lengths being connectedtogether end-to-end.
 13. An insulated pipeline comprising a multiplicityof insulated pipe structures, according to claim 1, the pipe structuresbeing joined end-to-end.
 14. An insulated pipeline according to claim13, wherein, at the connection of one pipe structure to another, athermally insulating arrangement is provided around the adjacent innerpipe ends between the adjacent connecting elements.
 15. An insulatedpipeline according to claim 13, in which the pipe structures arestructurally connected to one another at least principally by virtue ofthe connections of the ends of the inner pipes of the pipe structures.16. An insulated pipeline according to claim 15, in which the ends ofthe inner pipes of adjacent pipe structures are welded together.
 17. Amethod of forming an insulated pipe structure according to claim
 1. 18.A method according to claim 17, in which the outer shell comprises apre-formed tubular member and the connecting element is formed in situ.19. A method according to claim 17, in which the outer shell and theconnecting element are formed in situ during the same operation and areintegral with one another.
 20. A method of forming an insulated pipelineby joining together end-to-end a multiplicity of pipe structures, thepipe structures being according to claim
 1. 21. An insulated pipestructure according to claim 1, wherein the outer shell is made ofmetal.
 22. An insulated pipe structure according to claim 1, wherein theouter shell is made of steel.
 23. An insulated pipe structure suitablefor use in an undersea pipeline, the pipe structure being elongate andcomprising, intermediate its ends, an inner pipe for serving as aconduit for fluid flowing through the pipe structure, thermallyinsulating material extending around the inner pipe and a substantiallyrigid outer shell extending around the insulating material, wherein ateach of the ends of the pipe structure the inner pipe is connected tothe outer shell by a thermally insulating connecting element which issecured to the inner pipe at least partly by virtue of adhesive forcesbetween the inner pipe and the connecting element, the thermallyinsulating connecting element and the outer shell together protectingthe insulating material from water ingress and from ambient pressure,wherein the connecting element is formed as an integral part of theouter shell.
 24. An insulated pipe structure according to claim 23, inwhich the connecting element and the outer shell are formed in situ. 25.An insulated pipe structure suitable for use in an undersea pipeline,the pipe structure being elongate and comprising, intermediate its ends,an inner pipe for serving as a conduit for fluid flowing through thepipe structure, thermally insulating material extending around the innerpipe and a substantially rigid outer shell extending around theinsulating material, wherein at each of the ends of the pipe structurethe inner pipe is connected to the outer shell by a thermally insulatingconnecting element which is secured to the inner pipe at least partly byvirtue of adhesive forces between the inner pipe and the connectingelement, the thermally insulating connecting element and the outer shelltogether protecting the insulating material from water ingress and fromambient pressure, wherein the strength of the connection between theouter shell and the inner pipe is such that it can withstand alongitudinal shear force of 75 tonnes.
 26. An insulated pipe structureaccording to claim 25, wherein an outer surface of the inner pipe issurface-treated.
 27. A method of forming an insulated pipe structuresuitable for use in an undersea pipeline, the pipe structure beingelongate and comprising, intermediate its ends, an inner pipe forserving as a conduit for fluid flowing through the pipe structure,thermally insulating material extending around the inner pipe and asubstantially rigid outer shell extending around the insulatingmaterial, wherein the method includes: connecting the inner pipe to theouter shell at each of the ends of the pipe structure by a thermallyinsulating connecting element, wherein the thermally insulatingconnecting element is a unitary one piece element; and securing thethermally insulating connecting element to the inner pipe at leastpartly by adhesive forces between the inner pipe and the connectingelement; the thermally insulating connecting element and the outer shelltogether protecting the insulating material from water ingress and fromambient pressure.
 28. An insulated pipe structure suitable for use in anundersea pipeline, the pipe structure being elongate and comprising,intermediate its ends, an inner pipe for serving as a conduit for fluidflowing through the pipe structure, thermally insulating materialextending around the inner pipe and an outer shell extending around theinsulating material, wherein at each of the ends of the pipe structurethe inner pipe is connected to the outer shell by a unitary one-piecethermally insulating connecting element which is secured to the innerpipe at least partly by virtue of adhesive forces between the inner pipeand the connecting element, the connecting element engaging with anouter surface of the outer shell, the thermally insulating connectingelement and the outer shell together protecting the insulating materialfrom water ingress and from ambient pressure.
 29. An insulated pipestructure suitable for use in an undersea pipeline, the pipe structuringbeing elongate and comprising, intermediate its ends, an inner pipe forserving as a conduit for fluid flowing through the pipe structure,thermally insulating material extending around the inner pipe and asubstantially rigid outer shell, taking the form of a pre-formed tubularmember, extending around the insulating material, wherein at each of theends of the pipe structure the inner pipe is connected to the outershell by a unitary one-piece thermally insulating connecting element,the thermally insulating connecting element being arranged to engagewith an outer surface of the outer shell, the thermally insulatingconnecting element being secured to the outer shell predominantly byvirtue of adhesive forces between the outer shell and the connectingelement, the thermally insulating connecting element and the outer shelltogether protecting the insulating material from water ingress and fromambient pressure.
 30. An insulated pipe structure suitable for use in anundersea pipeline, the pipe structure being elongate and comprising,intermediate its ends, an inner pipe for serving as a conduit for fluidflowing through the pipe structure, thermally insulating materialextending around the inner pipe and a substantially rigid outer shellextending around the insulating material, wherein at each of the ends ofthe pipe structure the inner pipe is connected to the outer shell by aunitary one-piece thermally insulating connecting element which issecured to the inner pipe at least partly by virtue of adhesive forcesbetween the inner pipe and the connecting element, the outer shell beingintegrally formed with the connecting element, the thermally insulatingconnecting element and the outer shell together protecting theinsulating material from water ingress and from ambient pressure.
 31. Amethod of forming an insulated pipe structure suitable for use in anundersea pipeline, the pipe structure being elongate and comprising,intermediate its ends, an inner pipe for serving as a conduit for fluidflowing through the pipe structure, thermally insulating materialextending around the inner pipe and a substantially rigid outer shellextending around the insulating material, the method comprising:connecting the inner pipe at each of the ends of the pipe structure tothe outer shell by a thermally insulating connecting element, formed insitu, which is secured to the inner pipe at least partly by virtue ofadhesive forces between the inner pipe and the connecting element, andengages with an inner surface of the outer shell, the thermallyinsulating connecting element and the outer shell together protectingthe insulating material from water ingress and from ambient pressure,the strength of the connection between the outer shell and the innerpipe is such that it can withstand a longitudinal shear force betweenthe inner pipe and the outer shell of 75 tonnes.
 32. A method of formingan insulated undersea pipeline comprising a multiplicity of insulatedpipe structures connected together in end to end coaxial relationship,each pipe structure being elongate and comprising, intermediate itsends, an inner pipe for serving as a conduit for fluid flowing throughthe pipe structure, thermally insulating material extending around theinner pipe and a substantially rigid outer shell extending around theinsulating material, the method comprising: forming each pipe structureby connecting the inner pipe at each of its ends to the outer shell by athermally insulating connecting element which is secured to the innerpipe at least partly by virtue of adhesive forces between the inner pipeand the connecting element, the thermally insulating connecting elementand the outer shell together protecting the insulating material fromwater ingress and from ambient pressure, putting the pipe structures ona pipe laying vessel, forming the pipeline on the vessel by connectingthe inner pipes of the pipe structures to each other end to end, andlaying the pipeline into the sea from the vessel.
 33. A method accordingto claim 32, wherein some of the end to end connections of the pipestructures are made in advance of the laying of the pipeline and some ofthe end to end connections are made to the end of the pipeline as thepipeline is laid from the vessel.
 34. A method according to claim 32,wherein a pipe structure is made of a plurality of lengths of inner andouter pipes, wherein the pipe structure has at least one intermediatejoint.
 35. A method according to claim 34, wherein the at least oneintermediate joint is formed without the thermally insulating connectingelement.